1. Field of the Invention
The present invention relates generally to electrical measuring and testing apparatus and methods, and more particularly to apparatus and methods of fault location in cable systems.
2. Background Art
Two-way hybrid fiber coax (HFC) cable systems typically provide two-way communications for end users (typically homes or businesses) using both coaxial cable and fiber optic cable. A headend is a collection point for downstream signals and a termination point for upstream signals. “Downstream” or “forward” means signals traveling away from the headend and “upstream” or “reverse” means signals traveling toward the headend.
The coaxial portion of cable networks uses a tree-and-branch architecture to split downstream signals and combine upstream signals. On the coaxial portion of the cable plant, downstream signals are sent from a headend to an end user in a downstream frequency band, which may be 54 to 860 MHz. The composite downstream signal is typically comprised of analog television signals in the lower frequencies, such as 54 to 550 MHz, and digital television signals and cable modem traffic in the upper frequency band, such as 550 to 860 MHz. Upstream signals travel from the end users to the headend in the 5 to 42 MHz upstream frequency band over the same coaxial cable that is used for downstream communications.
The fiber portion of the plant is typically nearer to the headend and transports signals a long distance to a cluster of subscribers. The point at which the downstream fiber optic (light) signals are converted to downstream electrical signals for transmission over coaxial cable is called a fiber node. The upstream electrical signals are also converted into fiber optic signals at the fiber node for transmission back to the headend. In larger plants there may be additional signal distribution/collection points called “hubs”. In the United States the downstream is typically divided into 6 MHz channels that usually contain analog NTSC carriers or digital carriers. Normally, upstream channel spacing is not uniform.
Common path distortion (CPD) is an upstream impairment that is created on the coaxial portion of HFC cable systems. CPD is caused by downstream signals mixing together in non-linear elements to create an upstream interference that is comprised of distortion or inter-modulation products. CPD is typically produced by diodes that are formed by metallic corrosion in network elements such as taps, amplifiers, splitters, power inserters, and connectors. Finding the network element where the distortion is created is a difficult problem for cable technicians, because the act of touching or opening a network element frequently corrects the CPD problem, albeit temporarily.
With a large number of analog television channels (such as NTSC) on the downstream path, the upstream spectral plot caused by CPD has an appearance on a spectrum analyzer of three beats every 6 MHz across the return band. In the United States, with a standard frequency plan, second order distortion beats are centered every integer multiple of 6 MHz, and third order distortion beats are located every 1.25 MHz above and below the second order beats. If there are two radio frequency carriers at fa and fb, second order beats can be created by mixing products such as 2*fa, or 2*fb, or fa-fb, or fb-fa. If you have three carriers, such as fa, fb, and fc, third order beats can be created from many mixing products such 3*fa, 3*fb, 3*fc, fa+fb+bc, 2fa-fb, 2fb-fc, 2fc-fa, etc. The distortion products at the 6 MHz increments are second order distortion products because television channel spacing in the United States is 6 MHz. The third order distortion products at plus and minus 1.25 MHz from the second order beats are offset because video carrier frequencies, such as channel 2 at 55.25 MHz and channel 3 at 61.25 MHz, are not integer multiples of 6 MHz in a standard frequency plan.
While CPD has been observed on the upstream cable plant, mixing from CPD products also fall into the downstream frequency spectrum, where a visual impairment will be created that is similar to CSO (common second order) or CTB (composite triple beat), which are well-known downstream impairments. Thus, customer complaints about excessive downstream CTB may, in fact, be partially downstream CPD distortion products. Thus there is a need to find and fix sources of CPD.
The prior art method of finding CPD is to disconnect upstream network elements one at a time until the CPD goes away. This is a trial and error process, which is slow and prone to error because of the aforementioned problem of temporarily fixing CPD when the defective element is disturbed. Disconnecting network elements also disrupts services.
A new method in use by Optus in Australia ranges a distance to a CPD source using custom-designed special-purpose test equipment. This method is described by Australian patent application TW474071. Their system is comprised of PC-based test equipment, which injects test signals into vacant bandwidth on the downstream plant. On many cable systems vacant bandwidth for testing does not exist.
Interference with cable entertainment signals, voice carriers and data traffic for test is generally unacceptable.
This invention discloses a better method by determining a time delay associated with a defective CPD-creating element by using signals that are already being transported on the cable system. The time delay may be used to compute a round-trip distance using the known velocity of propagation. Knowing a distance to the CPD-creating element allows the defective device to be identified from a network map that shows distances.
This invention also discloses an improved injected test signal method that uses standard off-the-shelf test equipment.